The invention relates generally to a chip-like electronic component of a type designed for surface mounting, a so-called "Surface Mounted Device" or SMD.
Chip-like electrical components, sometimes referred to as leadless components and/or surface mounted devices, are well known in the prior art. One type of such devices is the solid electrolyte capacitor. The present invention, while not limited to such capacitors, will be described in terms of these devices.
Among the recent U.S. patents which describe solid electrolyte chip capacitors in some detail are U.S. Pat. Nos. 4,510,554 and 4,539,623, both assigned to Matsushita Electric Industrial Co., Ltd., and U.S. Pat. Nos. 4,571,664 and 4,660,127, both assigned to the assignee of this application. The latter patents are incorporated herein by reference.
Briefly, a solid electrolyte chip capacitor has a capacitor body including a porous valve metal anode member of very high surface area, usually of sintered tantalum, and an anode riser wire of the same valve metal extending from one end face thereof. A dielectric oxide film is thermally grown on the porous surfaces of the anode member. The pores are then filled with an intermediate oxide by impregnating the member with a solution of MnNO.sub.3, and then heating to form MnO.sub.2. The remaining surface area of the electrolyte-filled member (excluding the anode riser wire end face) is then covered with a carbon layer to provide electrical connection to the dielectric network, and finally coated with a conductive solderable material to provide an outer cathode layer. The capacitor body is then finished into its final form which may include an outer encapsulant of an insulating material.
The resulting SMD is leaded in such a way that it can be mounted and connected to a given circuit using a minimum amount of space on the circuit board. For this purpose, leads which extend a short distance away from the end faces of the device, and then descend parallel to and spaced apart from the end faces of said capacitor body, are preferred. The anode lead is welded to the riser wire, while the cathode lead is directly connected to the cathode terminal on the capacitor body. The ends of both leads are bent so as to be parallel to the mounting board, either outward away from the body, or preferably, to conserve space, inward. In the latter case, indents may be molded into the body to provide clearance for the leads. This preferred lead configuration will be referred to herein as the J-shaped configuration.
The lead constructions are designed not only to minimize mounting space, and thus maximize device density on the printed circuit board, but also to improve compliance of the device, both during mounting and afterwards.
In one method of mounting, a solder paste containing a flux is applied, such as by screen printing, to areas of the circuit pattern where the leads are to be attached. Next, the device is fixed in its proper location by adhering the body to the board using an organic adhesive such as a two-part epoxy or a u.v. curable resin. The board is then inverted and passed through a standing wave of molten solder, to achieve a solder joint. Finally, the board is washed in a solvent to remove the flux.
Because of differences in thermal expansion between the SMD and the board, the temperature cycling to which the assembly is subjected during soldering causes stresses on the solder joints. The J-shaped configuration of the leads, and their spaced apart relation to the device body, is intended to permit flexing during thermal cycling to relieve the stress and prevent degradation of the solder joints.
However, in practice, the molten solder tends to fill the spaces between the leads and the body of the device, reducing flex and leading to serious degradation or even failure of the solder joints.
Accordingly, it is an object of the invention to produce an SMD which can be reliably electrically connected to a printed circuit board.
It is another object of the invention to produce an SMD of the type known in the prior art, having leads spaced-apart from the body of the SMD, which leads can be reliably attached to the circuit pattern of a mother board such as a printed circuit board.
It is another object of the invention to produce an SMD of the type known in the prior art having leads with a J-shaped configuration, and spaced from the body of the SMD, which leads can be reliably attached to a mother board.